This invention relates to an eddy current imaging system for detecting flaws and for measuring property characteristics of materials. More specifically, this invention relates to a method of eddy current testing wherein a C scan of a spatial derivative of the amplitude and/or phase of a probe signal is provided in either real time, or for post processing analysis. Still more particularly, this invention relates to a method of determining scanned image sections with maximum ascent or descent of detected data and calculating an impedance plane trajectory for displaying an optimum flaw indication signal independent of scan pattern. Still more particularly, this invention relation to a method and apparatus for displaying a first spatial derivative of the instantaneous phase of the impedance of a signal generated by scanning a material with a selected eddy current sensing probe, in combination with a reconstructed optimum scan path calculated as a function of the regions of maximum ascent or descent of the data and an impedance plane trajectory.
A number of non-destructive testing techniques have been developed for detecting and displaying information regarding the structural integrity of a member. Such techniques have included eddy current testing methods for detecting flaws and measuring property characteristics. Systems utilizing such techniques are commercially available. In the case of flaw detection, the proximity of a material flaw to the test coil causes a change in the complex impedance of the coil which is detected and decoded by electronic circuitry in the system. Once the test parameters of frequency, coil size and orientation have been set, the response with an axisymmetric coil is nearly solely a function of the spatial relationship between the flaw and the test coil. Thus, for any set of surface coordinates x, y, the test response is calculable.
Most eddy current instruments provide either a strip chart or a complex impedance phasor plot of the detected data from the test coil to the instrument. In either case, the dynamic response of the test coil is a function of its position, as previously indicated, and the dynamics of the motion of scanning. Since both of these types of response are basically presentations of an instantaneous impedance of the coil as a function of time, the dynamic signal is thus a function of scan speed and scan direction for the test coil relative to the test specimen, and particularly with respect to a flaw. In general, therefore, the approach used for flaw detection is to scan in such a manner that coil motion is perpendicular to the major axis of a flaw. However, in the general case, this presumes some a oriori knowledge of the location and extent of the flaw orientation. An advantage of this scanning motion is that the maximum contrast signal, i.e. one having the highest signal to noise ratio, results from the perpendicular intersect of coil path and flaw outline which thus facilitates flaw detection.
Prior systems have also included a display of the complex impedance of the signal from the test coil as a function of the position of the coil rather than as a function of time. Such a C scan display, as such a display is commonly referred to, usually depicts the amplitude of a component of the impedance, although some prior art systems have also displayed phase in a C scan mode. In either case, the actual spatial position is usually inferred from the temporal data using an assumption of constant scan velocity and the plot of either signal amplitude or signal phase versus x-y position is indicated.
In particular, an instrument available from Zetec referred to in the art as a MIZ-18 instrument produces C scan plots of amplitude of the test signal from the coil, or a time derivative of the amplitude of the test signal from the coil.
However, the interpretation of such signals is still based on spatial information, even though the amplitude, phase, and time derivatives of the amplitude are useful. Such systems which use time derivatives are dependent on an assumption of constant velocity, as noted above, to deduce the spatial relationship of a detected flaw, especially in the presence of background noise. By using the assumption of constant velocity to deduce position, it has been determined in another context that the image sections with the maximum ascent or descent of the test signal is a significant method for detecting subsurface flaws.
Thus, it has remained a significant problem in this art to provide a system in which the mechanical scanning system and the data acquisition parameters can be determined somewhat independently so that the scanning can be done in any convenient format such as a meander pattern or by raster scanning without sacrificing the quality of the test results. As a practical example where such an approach would be beneficial is the inspection of a structure such as an airplane wing. Mechanically, the easiest way to scan the wing is in a meander scan along the length of the wing with incremental indexing steps at the end of each stroke. Typically, flaws would emanate from rivet holes at any arbitrary orientatin and the above scan would be expected to miss some flaws due to poor signal to noise ratio conditions at certain flaw orientations. With such a scan pattern system, the locally optimum scan direction could be determined and this scan synthesized from the available date. In this manner, the optimum mechanical and signal criteria can be simultaneously satisfied.
Accordingly, it has also remained a significant problem in this art to provide a reconstructed optimum scan path based on scan data wherein the optimum path traverses a flaw edge in a given pixel at a right angle, or near thereto, for maximizing the signal-to-noise ratio.